Ion implantation is generally performed to introduce an impurity into a semiconductor layer such as silicon. If the ion implantation is performed to introduce an impurity into a thin semiconductor layer, however, the impurity ions may also be introduced to other regions. Furthermore, uniformly distributing impurity ions over an inner surface of a hole with a very large aspect ratio by means of the ion implantation is difficult.
Introducing impurity ions to a semiconductor layer at the same time as forming the semiconductor layer may also be employed instead of performing the ion implantation after forming the semiconductor layer. A semiconductor layer with a relatively high impurity concentration may be obtained by this method. However, a dedicated machine for diluting an impurity to be introduced is needed to form a semiconductor layer with a relatively low impurity concentration by this method. This increases the manufacturing costs.